Nuclear Properties - doe-std-1128-98

DOE-STD-1128-98

National Council on Radiation Protection and Measurements (NCRP) have

recommended increasing quality factors to a value of 20 for fast neutrons (ICRP,

1985; NCRP, 1987a). Thus, it may be desirable to reduce neutron exposures.

Neutrons arise primarily from even-numbered plutonium isotopes (mostly 238Pu and

240Pu) as a result of spontaneous fission and alpha-neutron reactions with low-atomic-

number impurities in the plutonium. The 239Pu -enriched product of LIS will have

reduced concentration of these isotopes, resulting in lower intrinsic neutron

exposures. The LIS process can also result in significant reductions in gamma-ray

exposures for the product enriched in 239Pu. Much of the whole-body and most of the

extremity exposure is a result of surface contamination on the gloves and the interior

of the glovebox. The 241Am decay product, which results from the beta decay of 241Pu,

is a major contributor. Thus, the reduction of 241Pu can significantly reduce exposures

to hands and arms, as well as reduce the radiation streaming through glove ports in

shielded gloveboxes.

2.2

NUCLEAR PROPERTIES

Of the 15 plutonium isotopes, the two that have proven most useful are masses 239

and 238. Plutonium-239 is fissile, i.e., atoms of plutonium split upon exposure to

thermal or fast neutrons. Chemical reactions can release a few electron volts of

energy per atom; however, when a plutonium nucleus splits, it releases about 200

MeV of energy and two or three neutrons. This release of energy makes 239Pu useful

for nuclear weapons and reactor fuel. In fact, in light water reactors (LWRs) much of

the power originates from the fission of 239Pu, which is produced by neutron capture

in 238U. Because of its higher specific activity, 238Pu is used as long-lived heat sources

for powering planetary space missions where adequate solar energy is not available.

As mentioned before, all plutonium isotopes are radioactive. Isotopes with even mass

numbers (except mass number 246) are primarily alpha emitters. Isotopes of mass

numbers 232, 233, 234, 235, and 237 also decay by electron capture; isotopes of

mass numbers 241, 243, 245, and 246 decay by beta emission. Many of the alpha-

emitting isotopes, such as 238Pu and 240Pu, also fission spontaneously and emit

neutrons. All of the particle emissions are accompanied by X-ray and gamma-ray

emissions over a wide range of energies.

A review of the nuclear properties of plutonium (e.g., cross-sections, nuclear levels,

half-lives, and fission yields) can be found in Volume 1 of the Plutonium Handbook:

A Guide to the Technology (Wick, 1967) and in American National Standards

Institute (ANSI) Standard N317, Performance Criteria for Instrumentation Used for

In-Plant Plutonium Monitoring (ANSI, 1980a). Plutonium decay schemes, neutron

yields, and neutron energy spectra are described in the following sections.

2.2.1 Decay Schemes

The decay modes of some important plutonium and other isotopes and decay

products are shown in Table 2.3. For brevity, only the most abundant radiations

have been included in the table; more detailed information can be found in

papers by Gunnink and Morrow (1967) and Klein (1971), in ICRP Publication

38 (ICRP, 1983), and from the National Nuclear Data Center. Most of the

isotopes are strong alpha-emitters, making alpha heating a problem for the

storage and handling of large amounts of plutonium. The specific activities

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